My Minolta XTsi Maxxum has lost the cover to the battery compartment. Is a replacement piece available? I still love the camera.

Install fresh battery.

This camera, is a very modern electronic piece of equipment. It is important you use the correct batteries, put in the film correct and use the right settings, for the given light.
Would be great if you used the manual. I tries to fine one online, but found nothing.
My SLR camera's, with film, can't not be used without the knowledge how to set up them. I do have the manuals and still I have to use my manuals so now and than, because I do not use them that often.

ISO and aperture are most definitely not the same thing. ISO is the speed of the film. Aperture is the size of the opening in the lens, the larger the opening the more light comes through.

To change the aperture in Manual mode, hold down the exposure compensation button near the lens mount and turn the control dial on the front of the camera.

They're temporarily out of stock, but keep an eye on
http://resources.kodak.com/support/shtml/en/manuals/urg00576/urg00576c1s3.shtml

If your door to your battery compartment is broken, there are no parts left to repair it. Film cameras go for dirt cheap used. As long as the camera works, you could tape the door shut. The batteries should last for many months, so taping would be your only option.

The battery door on these models just hooks under a wire next to the battery hole. They can pop off rather easily as they are a small piece held in place by a spring, essentially.
You can often find the part on ebay for a few $ and snap it in place yourself. The same battery door is used by the XTsi, HTsi, STsi cameras.
Its very possible that you did not lock it properly the last time you put in new batteries. Hold the door down while you turn the quarter as you lock or unlock it. Otherwise the pressure of the battery spring will prevent a good lock.

LightspiritPhotography.com

Common problem with "aperture base"
Needs replacement.

I had a similar problem with my 8000i. Parts are getting scarce for these older Maxxums, but you can still get it repaired. Unless you are emotionally attached to this 7000i consider replacing it. You should be able to find another on Ebay for a decent price. You might also consider the subsequent generations which offer better AF performance. The 650xi, 700xi and 800xi are all good cameras and available on Ebay. Lighter versions like the Maxxum 70 and the XTsi are also available. The last of the film Maxxums was the single digit 7. It is generally regarded as the best of the Maxxum series, but still sells on Ebay over 150.00 typically. These subsequent models will take all your maxxum lenses and flashes. These newer models offer many other features and often less weight to carry around.

lightspiritphotography.com

http://uscamera.com/1min040.htm

You might try contacting Sony Parts dept. Sony purchased Minolta a few years ago and has the available parts for Minolta Maxxum cameras.